I have exact copies both on localhost and on the server (http://plugins_171.cmsimple.sk).
On localhost runs PHP7.1.8 on the server PHP7.0.0.
The problem is:
1. On localhost the pagemanager shows empty window (no pages are visible) and the Calendar 1.4.8 returns

Warning: A non-numeric value encountered in /Users/msereday/webpages/plugins_171/plugins/calendar/includes/eventlist.php on line 84
Warning: A non-numeric value encountered in /Users/msereday/webpages/plugins_171/plugins/calendar/includes/eventlist.php on line 84
Cannot modify header information - headers already sent (output started at /Users/msereday/webpages/plugins_171/plugins/calendar/includes/eventlist.php:84)

2. On the server I can't login getting

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator at support@exohosting.sk to inform them of the time this error occurred, and the actions you performed just before this error.
More information about this error may be available in the server error log.

Warning: A non-numeric value encountered in /Users/msereday/webpages/plugins_171/plugins/calendar/includes/eventlist.php on line 84
Warning: A non-numeric value encountered in /Users/msereday/webpages/plugins_171/plugins/calendar/includes/eventlist.php on line 84
Cannot modify header information - headers already sent (output started at /Users/msereday/webpages/plugins_171/plugins/calendar/includes/eventlist.php:84)

That's pretty strange. If a warning is issued, it shouldn't cause any output to be sent. Anyhow, let's fix the root cause: change this line to

Well, I have found some hints on the internet. Removing the .htaccess made no change. But now I have uploaded a celar 172 installation. It works fine. So I upload plugins one-one and hope to find the problem. Let you know, but it will take time with this ammount of plugins.

CMSimple.skIt's no shame to ask for an answer if all efforts failed.
But it's awful to ask without any effort to find the answer yourself.